Optical differentiation device

ABSTRACT

An optical device for the differentiation of a first optical signal and an optical signal lagging behind the first, in which the differentiation is represented by a signal carried by a continuous wave passing through the two arms of an interferometer including media with an index that depends on the optical power passing through them. The delayed signal is the first signal fed back to one of the arms through a delay.

FIELD OF THE INVENTION

This invention relates to the field of devices intended to create adifference signal between two optical signals. The invention isparticularly applicable to a device for reconstituting an optical signalfor transmission of data, and particularly a rectangular type signal,for example an encoded signal without a return to zero. The invention isalso applicable for the generation of a clock signal at twice thefrequency of the clock frequency of a first signal.

TECHNOLOGICAL BACKGROUND

An article by H. K. LEE et al entitled “All fibre optic clock recoveryfrom non return to zero format data” published in the “ElectronicsLetters” journal vol. 34 No. 5, March 1998 (document 1) describes adevice for reconstituting a clock signal starting from an optical signalfor transmission of data encoded in a code without a return to zero (NRZcode).

A part on the input side of the device described in this article is anoptical fibre differentiator that generates a signal with a pseudoreturn to zero (PRZ), starting from the signal in the NRZ code. The PRZsignal thus built up from the NRZ signal is then used in a known mannerto lock self-oscillating means. In the case described in the article, itis a laser cavity in locked mode comprising a non-linear optical loopmirror (NOLM).

The input side differentiator device comprises an asymmetric MachZehnder interferometric structure with two arms, one comprising a 300 psdelay τ in the form of a 6 cm additional fibre length. The NRZ signal isinput into each arm of the asymmetric Mach Zehnder interferometricstructure by means of a 3 dB coupler into which the NRZ signal is input.For the rest of the presentation, it is important to note that in theexperimental device described in this article, the NRZ signal wasgenerated in place by means of a tuneable laser diode in which thecontinuous output wave was modulated in a modulator into which an NRZmodulation signal output by a generator of this type of signal, wasinput. For a good understanding of the rest of the presentation, it isalso important to note that the delay τ, as described in the article incolumn 1 on page 479, represents the width of pulses forming thedifferential output signal at 3 dB. This delay τ must be equal to an oddnumber of half periods of the continuous carrier wave, if thedestructive condition for signals present in each of the arms of theasymmetric Mach Zehnder interferometric structure is to be satisfied. Toobtain this result, either the wavelength of this carrier must be variedas explained in the article at the top of column 2 on page 479 until thedestruction condition is obtained, or the fibre length causing the delaybetween the signals propagating in each of the arms, must be varied. Forobvious reasons of ease of construction, the authors chose a continuouswave generation diode, tuneable with sufficient resolution to obtain awavelength adjustment capable of creating a phase shift satisfying thedestruction condition.

The experimental device described in this article was used to obtain aPRZ signal starting from an NRZ signal at a rate of 1.5 Gigabits persecond. This PRZ signal is then used to lock a clock signalreconstituting the clock signal from the NRZ signal.

Note that in the experimental device described in this article, thewavelength of the signal carrier wave is available in place andtherefore that it is easy to act on it to adjust it and thus obtain thedestructive condition assuming a phase shift of (2k+1)π between thesignals circulating in each of the arms of the interferometer.

It is difficult to create an industrial application of the devicedescribed in this article, since in practice it is required toreconstitute the clock signal at a regenerator starting from a carrierwave of an NRZ signal for which the wavelength is not known in advance.Furthermore, the stability of the carrier wave may not be sufficient toguarantee the destruction condition in the long term. This is why thereis a need for a device capable of differentiating two signals, one ofwhich is delayed with respect to the other, in other words a device inwhich the delay between the two signals can be controlled to maintain anoperating difference that is equal to or close to (2k+1)π at all times.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, the problem of the phase adjustment betweenthe first and a second signal lagging behind the first signal, tosatisfy the destructive interference condition, is solved by a device inwhich there is a means for creating a continuous wave. This continuouswave is sent in a first channel comprising a medium with a refractionindex n that is variable as a function of a characteristic of thesignal, for example the frequency or the optical power passing throughthe medium. This same medium that has a refraction index n, is variableas a function of a characteristic of the signal, for example thefrequency or the optical power passing through it receives the firstsignal such that the index n of the medium is modulated by the high andlow levels of the characteristic of the first signal. This continuouswave is also sent in a second channel comprising a medium in which therefraction index n is variable under the same conditions. This samemedium in the second channel receives the second signal such that theindex n of the medium is modulated by the high and low levels of thesecond signal. For example, by modulating the power level of the firstand/or the second signal, the index n of the first and/or second mediumis modified, and therefore the time taken by the continuous optical wavepassing through these media, to pass through this medium, is modified.Therefore, the delay of one of the channels with respect to the othercan be adjusted to obtain a destructive beat between the first and thesecond signal. When the continuous wave that followed the first channeland the continuous wave that follow the second channel are made tointerfere, the phase shift between these two waves is equal to π, andthe difference between the wave modulated by the first signal and a wavemodulated by the second signal is determined. Therefore, this gives asignal representative of the difference between the first and secondsignals.

In summary, the invention relates to an optical device for thedifferentiation of two optical signals, a first signal and a secondsignal, the second signal being the same as the first signal but laggingbehind it by a delay τ comprising:

two channels, a first and a second channel, the first channel comprisinga delay means for delaying the first signal input into this channel byτ, the delayed signal forming the second signal,

means of generating a continuous wave,

device characterised in that it comprises:

a first medium and a second medium, with optical propagation indexesthat vary with a characteristic of the optical signal passing throughthe said medium, placed on the first and second channels respectively,

means of inputting firstly the first or the second signal and secondlythe continuous optical wave into the first medium, and of inputtingfirstly the first signal and secondly the continuous optical wave intothe second medium,

means of making the first and second signals output from the first andsecond media respectively interfere with each other, a signal present atthe output from these interference means making up the difference signalbetween the first and the second signal.

In the device according to the invention, the phase shift between thefirst and the second signal is independent of the wavelength of thesignal carrier wave.

Delay means delaying the first signal by τ may be placed indifferentlyon the input side or output side of the variable index optical medium.Considering the selected vocabulary convention, if the delay τ is on theinput side of the first medium the second signal is input into the firstmedium, whereas if the delay τ is on the output side of the first mediumthe first signal is input into the first medium.

Any means of multiplexing a signal input onto a channel and distributingit between two channels may be used to input the first signal onto thefirst and second channels, for example a 3 dB coupler or a multimodeinterferometric structure; the same is true for the interferometricoutput structure located on the output side of the first and secondmedia.

In the preferred embodiment, the index of the first and second mediavaries as a function of the optical power that passes through them andare optical semiconductor amplifiers. A phase delay adjustment isobtained by adjusting the polarization current of the amplifier,modifying the amplifier gain and therefore the power level passingthrough the optical medium. The variation of power passing through theoptical medium causes a variation of the index of this medium. Thus, itcan be seen that the adjustment of the gain creates a variation of thepropagation time. In this embodiment, the phase delay is preferablycontrolled in a closed loop in order to minimise the average level ofthe difference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the invention will become clear after reading thedescription of a preferred embodiment and variants that will be madebelow with reference to the attached drawings in which:

FIG. 1 represents: in part A, an example shape of the first signal, inpart B, the signal of the delayed part A, in part C, the differencesignal between the signals in parts A and B, in part D, a clock signalreconstituted from the pseudo clock signal represented in part C;

FIG. 2 is a diagrammatic view of a device according to the invention;

FIG. 3 is a diagrammatic view of a device according to the invention inwhich multimode interferometers act as the input and output structuresin and outside the first and second channels respectively;

FIG. 4 is an example of a device for reconstituting a clock signal, forexample an NRZ transmission signal.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 is intended to explain what is meant by a difference signalbetween the first signal and the same delayed signal. Parts A and B inFIG. 1 represent the envelope of a signal and of the same delayed signalrespectively. For example, it may be an NRZ transmission signal. Thesesignals are carried by an optical carrier wave (not shown) with a veryshort wavelength compared with the period of the carried signal. Whenthe delay between the first and second signals is less than the periodof the carried signal, the difference signal includes a pulse as drawnin part C, with a duration that is equal to the delay, every time thatthe signal in part A includes a rising front or a falling front, inother words for a digital signal, every time that there is a change from0 to 1 or from 1 to 0. In this case, pulses of the difference signalshown in part C represent clock pulses of the NRZ signal. These pulsesmay be used to lock a self-oscillating device, for example a laser fibrecavity in locked mode as described in the document mentioned above, or aself-oscillating diode.

FIG. 2 represents an example of device 1 according to the invention. Thedevice comprises two channels 5 and 6. A continuous optical wavegenerated by a continuous wave generator 2, for example a laser diode,is coupled through a coupling means 4, for example a 3 dB coupler toeach of channels 5 and 6 respectively. The first signal, for examplecarried by an optical fibre, is also coupled through a coupling means 3,for example a 3 dB coupler, to each of channels 5 and 6 respectively.The first channel 5 comprises a delay means 7 in series with a medium 10with a propagation index n that is variable as a function of the opticalpower passing through it. The second channel 6 comprises a medium 11 forwhich the propagation index n is variable as a function of the opticalpower that passes through it. In the preferred embodiment, the media 10and 11 are composed of optical semiconductor amplifiers. In thispreferred embodiment, the delay 7 and the optical semiconductoramplifier 10 for the first channel 5 were both produced on the samecomponent. In the experimental device made in the laboratory, a HeinrichHertz Institute (HHI) (BERLIN-DE) component was used comprising anasymmetric Mach Zehnder interferometer comprising two opticalsemiconductor amplifiers, four input guides and four output guides. Onlythe inputs and outputs used are shown in FIG. 2. This device generates a7 picosecond delay that was found to be sufficient to obtain adifference signal, for example starting from a 2.5 Gigabit transmissionsignal with a bit period of 400 picoseconds or a 10 Gigabit transmissionsignal with a bit period of 100 picoseconds.

It was already mentioned above with regard to the description in FIG. 1,that the delay should be less than the period of the carried signal, inother words in the two cases mentioned immediately above, less than 400and 100 picoseconds respectively. The bit time period is the maximumduration of the delay, its minimum duration has not been explored. Thedelay duration represents the pulse width resulting from thedifferentiation. These pulses should be perceptible. Work done by theapplicant has shown that a delay of between about 7 picoseconds and theduration of the bit time may be suitable.

The delay means 7 may also be made using an additional fibre length onthe first channel. In this case, the fibre carrying the signal on thefirst channel 5 is longer than the fibre carrying the signal on thesecond channel 6. This increased length is measured between the outputsof the coupler 3 and the input to an interferometric structure 8 intowhich signals on the output side of media 10 and 11 are input. The delay7 may also be in the form of a specific component 7, and in this case itmay be placed on the first channel 5 on the output side of the medium 10or as shown in dotted lines on the input side of medium 10, whileremaining on the output side of the point at which the continuous wavefrom the continuous wave generator 2 enters this channel. It may also beon the input side of this entry point of the continuous wave from thecontinuous wave generator 2, as is also shown in dashed lines.

Means 9 of adjusting a polarisation current can be used to adjust theamplification level of the optical amplifier 10, and also the crossingtime through an optical medium installed in a known manner in thisamplifier. In the preferred embodiment, the adjustment is made in aconstant and automatic manner by means of a closed loop regulationtending to minimise the average value of an optical signal representingthe difference signal directly at the output from the interferometricstructure 8 or preferably on the output side of a filter 23 centred onthe wavelength of the continuous wave from the continuous wave generator2. The loop comprises an optical power detector 22 detecting the powerat the output from filter 23. For example, this detector 22 may be aphotodiode followed by an integrator circuit. We will now describeanother embodiment with reference to FIG. 3. The embodiment shown inthis figure is different from the previous embodiment due to the meansof entry of the first signal and the continuous wave on channels 5 and6. A multimode interferometer 13, for example a Fabry Perotinterferometer with two inputs, a first input 14 and a second input 15.The signal is input into the first input 14. The continuous wave fromthe continuous wave generator 2 is input into the second input 15. Twooutputs, a first output 16 and a second output 17 from the multimodeinterferometer 13 each receives the continuous wave from the continuouswave generator 2, and the signal. The first output 16 and the secondoutput 17 are coupled to the first channel 5 and the second channel 6respectively. The delay 7 is placed on the input or output side of themedium 10 on channel 5. Similarly, the output 19 from channel 5 and theoutput 20 from channel 6 are coupled to a first input 19 and a secondinput 20 to a multimode output interferometer 18. The inputs to thisinterferometer are marked with the same references 19, 20 as the outputsfrom channels 5 and 6, since the outputs from channels 5 and 6 also formthe inputs to the multimode interferometer 13.

It will be noted that in the embodiments shown in FIGS. 3 and 4, thefirst and second channels comprise a first medium 10 and a second medium11 respectively. Also, they may comprise or not comprise the delay 7,depending on embodiment variants. When channels 5 and 6 do not comprisethe delay 7, channels 5 and 6 and the interferometric structure 8 or 18together form a symmetric interferometric Mach Zehnder structure, thestructure is asymmetric when the delay 7 is present on the channel 5 onthe output side of the inputs of the signal and the continuous wave onthis channel.

The operation of the device was already described above. The continuouswave and the first or second signal is input into the medium 10, forwhich the propagation index n is variable as a function of the opticalpower passing through it, such that the index n of the medium 10 ismodulated by the high and low levels of the first signal. The continuouswave is also sent into the medium 11 in which the propagation index n isvariable as a function of the optical power passing through the medium.This same medium 11 of the second channel receives the first signal suchthat the index n of the medium is modulated by the high and low levelsof the first signal. By modifying the power level of the signal passingthrough the first medium 10, the index n of the first medium, andtherefore the time taken by the continuous optical wave to pass throughthis medium 10, can be modified. Therefore, this provides a means ofadjusting the phase shift of one of the channels with respect to theother to obtain a destructive beat between the first and the secondsignals. When the continuous wave that followed the first channel andthe continuous wave that followed the second channel are made tointerfere, there is a phase shift equal to n between these two waves,and the difference between a wave modulated by the first signal and awave modulated by the second signal is determined every time that thepower levels on media 10 and 11 are identical. Therefore, this gives asignal representative of the difference between the first and the secondsignal at the output from the interferometric structure 8 or 18. Thissignal may be used to create a signal with a constant period such as aclock signal, preferably after being filtered by filter 23.

This type of device 30 for creating a clock signal from an opticaldifferentiation device 1 like that shown in FIG. 2 or 3, is shown inFIG. 4. The clock signal creation device 30 comprises a device 1according to one of the embodiments of the invention. The differencesignal at the output from this device 1 is received by self-oscillatingmeans 31. For example, it may be a laser fibre cavity in locked mode ora self-oscillating diode. A self-oscillating HHI diode was used in theexperimental embodiment. When the signal at the input to device 1according to the invention is an NRZ transmission signal, the differencesignal at the output from the device 1 is a pseudo signal with return tozero (PRZ). When this PRZ signal is input into the self-oscillatingdevice 31, the clock signal of the NRZ signal is obtained at the outputfrom the self-oscillating device 31 in a known manner.

When the signal at the input to device 1 according to the invention is aclock signal, the difference signal at the output from device 1according to the invention is a clock signal with a frequency equal totwice the frequency of the input clock signal, since there is one outputpulse for each rising or falling front of the input signal. The width ofeach clock pulse is an increasing function of the delay T between thetwo waves.

Note that when the input signal is a clock signal, a clock signal withtwice the frequency is obtained directly without a self-oscillatingdevice 31.

Thus, the device 1 according to the invention may be used to obtain aclock signal with a frequency equal to twice the frequency of an inputsignal composed of a first clock signal.

What is claimed is:
 1. An optical device for the differentiation of twooptical signals, a first signal and a second signal, the second signalbeing the same as the first signal but lagging behind it by a delay τ,comprising: a first channel and a second channel, said first channelcomprising a first medium having an optical index and a second channelcomprising a second medium having another optical index, wherein theoptical indexes of each of said first and second mediums vary with acharacteristic of an optical signal passing through them: means ofgenerating a continuous wave; means of making a signal present on thefirst channel interfere with a signal present on the second channel, anoutput signal from these interference means forming a difference signalbetween the first and the second signal; means of inputting thecontinuous wave on each of the two channels, wherein the first channelalso comprises: a delay means placed in series with the first medium fordelaying said first signal; and a means of inputting the first signal oneach of the channels, the input onto the first channel taking place onthe input side of the series formed by the first medium and the delaymeans.
 2. Device according to claim 1, wherein an output from the meansof inputting the continuous wave onto the first channel is located onthe output side of the delay means τ.
 3. Device according to claim 1,wherein an output from the means of inputting the continuous wave ontothe first channel is located on the input side of the delay means τ. 4.Device according to claim 3, wherein the means of inputting the firstsignal onto the first channel and onto the second channel comprise amultimode interferometric input structure with two inputs of a firstinput and a second input and two outputs of a first output and a secondoutput, the first signal being applied to the first input and thecontinuous wave being applied to the second input, the first outputbeing coupled to the first channel and the second output being coupledto the second channel.
 5. Device according to claim 3, wherein the meansof making signals output from the first channel and the second channelinterfere comprise a multimode interferometric output structure with twoinputs of a first input and a second input, and an output, the firstinput being coupled to the first channel, the second input being coupledto the second channel, the output from this multimode interferometricoutput structure forming the output carrying the difference signal. 6.Device according to claim 1, further comprising means of adjusting theoptical propagation index of a medium with an index that varies with theoptical power of the optical signal passing through the said medium,said means acting on the medium with a variable index in at least one ofthe channels.
 7. Device according to claim 6, wherein the first mediumand second medium have an optical refraction index that varies as afunction of the optical power passing through it, are opticalsemiconductor amplifiers, and the means of adjusting the optical indexis composed of means of varying the value of a polarization current ofat least one of said amplifiers.
 8. Device according to claim 7, furthercomprising detection means on the output side of the output carrying thedifference signal, to detect the optical power of the difference signal,an electric signal present at the output of these means being coupledand fed back onto the means of adjusting the polarization current of atleast one of the optical amplifiers in order to minimize the value ofthis electric signal.
 9. Device according to claim 1, wherein the firstsignal is a digital data signal with one bit duration, and the means ofintroducing the delay τ introduces a delay with a duration between about7 picoseconds and the bit duration.
 10. Device according to claim 9,wherein the delay duration introduced by the means of introducing thedelay τ is about 7 picoseconds.
 11. A device for reconstitution of aclock signal of an optical data transmission signal, comprising: anoptical device for differentiation of two optical signals, a firstsignal and a second signal, the second signal being the same as thefirst signal but lagging behind it by a delay τ, comprising: a firstchannel and a second channel, said first channel comprising a firstmedium having an optical index and a second channel comprising a secondmedium having another optical index, wherein the optical indexes of eachof said first and second mediums vary with a characteristic of anoptical signal passing through them; means of generating a continuouswave; means of making a signal present on the first channel interferewith a signal present on the second channel, an output signal from theseinterference means forming a difference signal between the first and thesecond signal; means of inputting the continuous wave on each of the twochannels, wherein the first channel also comprises: a delay means placedin series with the first medium for delaying said first signal; and ameans of inputting the first signal on each of the channels, the inputonto the first channel taking place on the input side of the seriesformed by the first medium and the delay means, and a triggered opticalself-oscillating means located on the output side of the opticaldifferentiation device for two optical signals, wherein the differencesignal from the optical device for differentiation of the two opticalsignals is applied to the self-oscillating optical means, and outputtingthe reconstituted clock signal, the first signal in this case being theoptical transmission signal.
 12. The device for reconstitution of aclock signal of an optical data transmission signal according to claim11, wherein the self-oscillating optical means comprises aself-oscillating laser diode.
 13. A method of using the opticaldifferentiation device for two optical signals according to claim 6, tocreate a clock signal with a frequency equal to twice the frequency of afirst clock signal.